Dynamics of Chemical Reactions and Petrophysical Property Variations in Shallow Carbonate Aquifers During CO2 Injection
Abstract
Carbonate reservoirs are attractive targets for geological CO2 storage due to their high storage capacity. However, CO2 injection can induce complex geochemical reactions that alter reservoir petrophysical properties in unpredictable ways. This study presents laboratory injectivity tests using CO2-saturated brine (i.e., carbonated brine) on five carbonate core samples with varying lithologies and pore structures, arranged in series in a single core stack. We evaluated changes in porosity and permeability, monitored inlet–outlet differential pressure, analyzed effluent chemistry (Ca2+, Mg2+, and Fe2+), and examined the changes in pore structures using micro-computed tomography (micro-CT). Calcite-rich limestones showed the largest increases in porosity and permeability, consistent with calcite dissolution, while dolomite-rich samples showed comparatively small and mixed-sign changes. Differential pressure across the stack evolved through three stages: (1) a slow pressure rises over the first ~17 pore volumes (PV); (2) a rapid pressure drops over the next ~15 PV; and (3) a prolonged, more gradual decline over the remaining injection. Whole-stack water-based permeability, back-calculated from Darcy’s law, decreased from ~0.03 mD to ~0.018 mD during Stage I and then recovered to ~0.09 mD by the end of the test (82 days, 57 PV total). Effluent analysis supported these trends: Ca2+ levels dropped from 97.2 ppm to 20 ppm during precipitation, then rose to 415 ppm with dissolution. Dissolved Mg2+ remained stable, while Fe2+ briefly spiked due to ferroan mineral dissolution. Micro-CT (25 μm resolution) imaging confirmed pore enlargement and fracture widening in the limestone samples and showed no resolvable change in the dolostones. These results, together with the limitations we identify, are intended to inform the design of future carbonate CO2 injectivity experiments and storage strategies.